1. Field of the Invention
The present invention relates to a wireless communication station and, more particularly, to an access point which communicates with wireless communication stations via a plurality of channels.
2. Description of the Related Art
There is known a wireless LAN system making communication between an access point (AP) and a plurality of wireless communication stations (STAs). As this wireless LAN system, there is provided a wireless LAN system (ISO/IEC 8802-11:1999(E) ANSI/IEEE std 802.11, 1999 edition) under the IEEE802.11 standard that defines a CSMA (Carrier Sense Multiple Access). In this wireless LAN system, in general, prior to transmitting a packet to a wireless station, which is so called a target station to which the packet is to be transmitted, an access point senses a carrier for transmitting the packet. However, where a reception state in which a packet including channel reservation information is received from another station is established, packet transmission to the target station is suspended. Where the transmission is suspended, the access point starts transmission of a packet for specifying an address for the target station after ending of the packet reception state for the another station and elapse of a random transmission waiting period computed at a control section of the access point. In this way, processing in which the transmission to the station is suspended, the random transmission waiting period is elapsed, and the packet is transmitted is referred to as back-off processing.
The target station transmits an acknowledge response (ACK) packet to the access point after a predetermined period if data on the received packet has been normal. At the access point, when the ACK packet from the target station cannot be received after the elapse of the predetermined period, the control section executes back-off processing, and a packet is transmitted again.
On the other hand, a Space Division Multiple Access (SDMA) method is known as one of the multiplexing schemes in a wireless communication system. In the SDMA method, a plurality of antenna beams for spatially decreasing mutual interference are generated from an adaptive array antenna provided at the access point. Therefore, the SDMA method can improve its communication quality and can achieve simultaneous communication between the access point and a plurality of stations. The SDMA method is applied to a wireless LAN system of the CSMA method, whereby it is expected that an advantage of the SDMA method can be provided.
However, if the SDMA method is simply applied to the wireless LAN system of the CSMA method, the following problem occurs.
In the SDMA method, at the access point, there is provided transmitter and receiver modules corresponding to a plurality of antenna beams formed by an adaptive array antenna. Further, individual control sections control the transmitter and receiver modules, respectively. In an ideal communication system provided with the adaptive array antenna, each of the antenna beams can be used for receiving only one signal from the target station without being affected from another stations. Thus, if the CSMA is executed for each antenna beam by these individual control sections, back-off processing is executed for each antenna beam. As a result, there can occur a case in which, at the access point, both of transmission to one station and reception from another station takes place at the same time. That is, even if the access point attempts to receive a packet from one station (TE1), when transmission to another station (TE2, TE3) takes place at the same time as the reception, such transmission may act as an interference and affect on the reception from the station TE1, thus disabling reception.
Where the access point transmits packets to the plurality of stations (TE1, TE2, TE3), no reception state occurs at the access point. Thus, a collision between transmission and reception can be basically prevented. However, when one side of packet to be transmitted to one station TE1 is smaller than that of packet to be transmitted to another station TE2, TE3, even if the access point is placed in a transmitting state for transmitting the packets to the stations TE2, TE3, respectively, the station TE1 generally terminates packet reception. Therefore, this station TE1 transmits an ACK packet to the access point. As a result, even if the access point could receive the ACK packet from the station TE1, the access point may not actually receive the ACK packet from the station TE1 due to an interference caused by the packet transmission from the access point to the stations TE2, TE3. This ACK packet has an effect on transmission to the station TE2, TE3, and the station TE2, TE3 cannot receive a packet from the access point.
Further, at the access point, where an ACK packet is not received from one of a plurality of stations TE1, TE2 and TE3, for example, one station TE2, there is a problem that back-off processing is executed at the access point for transmission and reception between the access point and the station TE2, and a timing of transmitting a packet to each of the stations TE1, TE2 and TE3 during this processing and subsequent is shifted.
The similar problem will occur in the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method in which communication is autonomously controlled by using a protocol as well as a case of simply applying the SDMA (Space Division Multiple Access) method to the wireless LAN system of the CSMA (Carrier Sense Multiple Access) method. That is, in the wireless access point in which the CSMA/CA method has been employed, where a channel to be used has been an adjacent channel or the same channel, there is a problem that signals of the respective channels interfere with each other, and a data transmission error occurs.